Various MEMS devices are becoming increasingly popular. MEMS transducers, and especially MEMS capacitive microphones, are increasingly being used in portable electronic devices such as mobile telephone and portable computing devices.
Microphone devices formed using MEMS fabrication processes typically comprise one or more moveable membranes and a static backplate, with a respective electrode deposited on the membrane(s) and backplate, wherein one electrode is used for read-out/drive and the other is used for biasing, and wherein a substrate supports at least the membrane(s) and typically the backplate also. In the case of MEMS pressure sensors and microphones the read out is usually accomplished by measuring the capacitance between the membrane and backplate electrodes. In the case of transducers, the device is driven, i.e. biased, by a potential difference provided across the membrane and backplate electrodes.
FIGS. 1a and 1b show a schematic diagram and a perspective view, respectively, of a known capacitive MEMS microphone device 100. The capacitive microphone device 100 comprises a membrane layer 101 which forms a flexible membrane which is free to move in response to pressure differences generated by sound waves. A first electrode 103 is mechanically coupled to the flexible membrane, and together they form a first capacitive plate of the capacitive microphone device. A second electrode 102 is mechanically coupled to a generally rigid structural layer or backplate 104, which together form a second capacitive plate of the capacitive microphone device. In the example shown in FIG. 1a the second electrode 102 is embedded within the backplate structure 104.
The capacitive microphone is formed on a substrate 105, for example a silicon wafer, which may have upper and lower oxide layers 106, 107 formed thereon. A cavity or through-hole 108 in the substrate and in any overlying layers (hereinafter also referred to as a substrate cavity) is provided below the membrane, and may be formed for example using a “back-etch” through the substrate 105. The substrate cavity 108 connects to a first cavity 109 located directly below the membrane. These cavities 108 and 109 may collectively provide an acoustic volume thus allowing movement of the membrane in response to an acoustic stimulus. Interposed between the first and second electrodes 102 and 103 is a second cavity 110.
A plurality of holes, hereinafter referred to as bleed holes 111, connect the first cavity 109 and the second cavity 110.
A further plurality of holes, hereinafter referred to as acoustic holes 112, are arranged in the back-plate 104 so as to allow free movement of air molecules through the back plate, such that the second cavity 110 forms part of an acoustic volume with a space on the other side of the back-plate. The membrane 101 is thus supported between two volumes, one volume comprising cavities 109 and substrate cavity 108 and another volume comprising cavity 110 and any space above the back-plate. These volumes are sized such that the membrane can move in response to the sound waves entering via one of these volumes. Typically the volume through which incident sound waves reach the membrane is termed the “front volume” with the other volume, which may be substantially sealed, being referred to as a “back volume”.
In some applications the backplate may be arranged in the front volume, so that incident sound reaches the membrane via the acoustic holes 112 in the backplate 104. In such a case the substrate cavity 108 may be sized to provide at least a significant part of a suitable back-volume.
In other applications, the microphone may be arranged so that sound may be received via the substrate cavity 108 in use, i.e. the substrate cavity forms part of an acoustic channel to the membrane and part of the front volume. In such applications the backplate 104 forms part of the back-volume which is typically enclosed by some other structure, such as a suitable package.
It should also be noted that whilst FIG. 1 shows the backplate 104 being supported on the opposite side of the membrane to the substrate 105, arrangements are known where the backplate 104 is formed closest to the substrate with the membrane layer 101 supported above it.
In use, in response to a sound wave corresponding to a pressure wave incident on the microphone, the membrane is deformed slightly from its equilibrium position. The distance between the lower electrode 103 and the upper electrode 102 is correspondingly altered, giving rise to a change in capacitance between the two electrodes that is subsequently detected by electronic circuitry (not shown). The bleed holes allow the pressure in the first and second cavities to equalise over a relatively long timescales (in acoustic frequency terms) which reduces the effect of low frequency pressure variations, e.g. arising from temperature variations and the like, but without significantly impacting on sensitivity at the desired acoustic frequencies.
One skilled in the art will appreciate that MEMS transducers are typically formed on a wafer before being singulated. Increasing it is proposed that at least some electronic circuitry, e.g. for read-out and/or drive of the transducer, is also provided as part of an integrated circuit with the transducer. For example a MEMS microphone may be formed as an integrated circuit with at least some amplifier circuitry and/or some circuitry for biasing the microphone. The footprint of the area required for the transducer and any circuitry will determine how many devices can be formed on a given wafer and thus impact on the cost of the MEMS device. There is therefore a general desire to reduce the footprint required for fabrication of a MEMS device on a wafer.
In addition to be suitable for use in portable electronic devices such transducers should be able to survive the expected handling and use of the portable device, which may include the device being accidentally dropped.
If a device such as a mobile telephone is subject to a fall, this can result not only in a mechanical shock due to impact but also a high pressure impulse incident on a MEMS transducer. For example, a mobile telephone may have a sound port for a MEMS microphone on one face of the device. If the device falls onto that face, some air may be compressed by the falling device and forced into the sound port. This may result in a high pressure impulse incident on the transducer. It has been found that in conventional MEMS transducers high pressure impulses can potentially lead to damage of the transducer.
To help prevent any damage which may be caused by these high pressure impulses it has been proposed that the MEMS transducer could be provided with variable vents which can provide a flow path between the front and back volumes that has a size that can vary in use. In a high pressure situation the variable vent(s) provide a relatively large flow path between the volumes so as to provide for relatively rapid equalisation between the volumes, reducing the extent and/or duration of a high pressure event on the membrane. At lower pressures however, within the expected normal operating range of the transducer, the size of the flow path, if any, is smaller.
The variable vent structure thus acts as a type of pressure relief valve to reduce the pressure differential acting on the membrane at relatively high pressure differentials. However unlike, the bleed holes which may be present in the membrane which have a fixed area and thus a fixed size of flow path, the variable vent has a flow path size which varies in response to a pressure differential. Thus the degree to which the variable vent allows venting depends on the pressure differential acting on the vent—which clearly depends on the pressure of at least one of the first and second volumes. The variable vent therefore provides a variable acoustic impedance.
It will be appreciated that, in the membrane layer of a MEMS transducer, a material is said to be under stress when its atoms are displaced from their equilibrium positions due to the action of a force. Thus, a force that increases or decreases the interatomic distance between the atoms of the membrane layer gives rise to stress within the membrane. For example, the membrane layer exhibits an inherent, or intrinsic, residual stress when at equilibrium (i.e. when no or negligible differential pressure arises across the membrane). Furthermore, stresses can arise in the membrane layer e.g. due to the way in which the membrane is supported in a fixed relation to the substrate or due to an acoustic pressure wave incident on the membrane.
MEMS transducers according to the present invention are intended to respond to the acoustic pressure waves which give rise to transient stress waves on the membrane surface. Thus, it will be appreciated that the stresses exhibited within a membrane layer, both when at equilibrium and when moving during use, may potentially have a detrimental impact on the performance of a transducer, as described below.
FIG. 2 shows a cross-sectional view through a typical transducer structure. The transducer structure comprises a membrane 101 which is moveable during use in relation to a rigid backplate 104. The membrane 101 and backplate 104 are supported by a substrate 105, the substrate 105 comprising a cavity or though-hole 108. Electrodes and other features are not shown in FIG. 2 for clarity purposes.
Referring to FIG. 3, during movement of the membrane 101 during use, and in particular during high input acoustic pressure, or extreme conditions such as a mobile device being dropped, it is possible that the membrane 101 makes contact with the substrate 105 which provides support for the membrane. For example, the membrane 101 can make contact with a peripheral edge of the substrate 105 that forms the cavity within the substrate, as illustrated by the arrow 30.
Embodiments of the present invention are generally concerned with improving the efficiency and/or performance of a transducer structure. Aspects of the present invention are also concerned with alleviating and/or redistributing stresses within the membrane layer, including when a membrane moves or flexes during use.
Aspects of the present invention are also directed to alleviating and/or diffusing and/or redistributing stress arising in the membrane if the membrane is displaced during use such that the membrane makes contact with the substrate which supports the membrane.